The central hypothesis to be tested is that the initiation and maturation of angiotensin II (AngII)-induced abdominal aortic aneurysms (AAAs is mediated by a specific matrix metalloproteinase (MMP) that is secreted and/or activated via a leukocyte-specific mechanism. To test this hypothesis, we propose the following aims: Aim 1. Determine whether a specific MMP is responsible for AngII- induced AAA.. To identify the action of a specific MMP, we will use mice with genetically engineered deficiencies of specific MMPs. We will focus our efforts on MMP-2, -9, and -12, based on the demonstration of their presence in aneurysmal tissue and their previous inferred roles in the disease process. Further, we will define the temporal sequence of elaboration of MMPs and their endogenous inhibitors in evolving and mature aneurysmal tissue. Aim 2. Determine whether AngII induces the release of MMPs from leukocytes and/or enhances the activation of MMPs secreted by neighboring vascular cells. To determine the direct contribution of AngII on MMP mediated AAA formation, we will perform bone marrow transplantation studies in which bone marrow cells from MMP+/+ or -/- mice will be used to repopulate MMP+/+ recipient mice. Aim 3. Determine the mechanism of AngII on leukocyte-dependent regulation of MMP activity. We will determine whether AngII exerts a direct effect on the synthesis and secretion of specific MMPs in cultured cells. Further, we will define whether AngII results in increased activation of MMPs. Aim 4. Determine the effects of MMP inhibition when selective inhibitors are administrated after the formation of AAAs. In preliminary studies we demonstrated that inhibition of MMPs prevented the initiation of AngII-induced AAA. However, in the human disease ultrasound is used to detect an already formed AAA, and thus an effective pharmacologic treatment would have to either reduce the progression or reverse the pathology of an established AAA. It is conceivable that MMPs play a different role in the remodeling process in established AAAs, as compared to their role in the initiation of the disease. Therefore, initial studies will determine the effects of broad specificity MMP inhibitors on the maturation process of an already formed AAA. Subsequent studies will use more selective inhibitors of MMPs based on data obtained in Specific Aims 1-3. The significance of this research relates to delineation of mechanism for AA formation and maturation, with emphasis on potential development of pharmacological treatments of the disease.